To achieve NO.sub.x control in lean burn engines, exhaust after-treatment systems have included an additional three-way catalyst, often referred to as a lean NO.sub.x trap (LNT). Presently, however, the performance of NO.sub.x trap technology is limited in several respects. NO.sub.x trap performance is affected by the operating temperature and requires a relatively narrow temperature-operating window. At temperatures outside this window, the device will not operate efficiently and NO.sub.x emissions will increase. Exposure to high temperature will also result in permanent degradation of the NO.sub.x trap capacity.
A current approach to controlling the temperature of the exhaust gas entering aftertreatment devices implements a variable geometry exhaust system having two exhaust paths. When the temperature is high, a valve sends the exhaust gas down a long path from the engine to the aftertreatment device, which provides additional cooling primarily by convection. Otherwise, the exhaust gas follows a shorter path that provides considerably less cooling. This system has limited flexibility, and is difficult to package in a limited space. It also adds cost and complexity to the emissions system, without significant additional benefits.
Governmental regulations of emissions are becoming more stringent. The efficiency of catalysts is temperature dependent. Thus, there exists a need for close control of the temperature of an exhaust after-treatment system to provide an effective means of reducing undesirable emissions.